This invention relates to fuel cell assemblies with ion exchange membrane cells such as taught in U.S. Pat. No. 4,175,165 by the first-named inventor herein. More particularly, this invention provides cell assemblies with improved water and thermal management.
It has been known for some time that fuel cell assemblies with ion exchange membrane cells can be suitable sources of electric power. They may be started up at or below room temperature and require no corrosive electrolytes for operation. A hydrogen-oxygen reaction is employed for power generation, so water is the reaction product and thus the only liquid to be dealt with.
Hydrogen is a viable fuel choice for cells with ion exchange membrane electrolytes. Hydrogen can be obtained from known sources such as methanol. However, hydrogen stored as a reversible metal hydride is especially attractive. Typical reversible metal hydride formers include LaNi.sub.5, FeTi and MmNi.sub.4.15 Fe.sub.0.85 among others. Storage density of hydrogen in hydrides is abundant and provides an attractive alternative to other modes of hydrogen supply, from the standpoint of safety. Hydrogen release from hydride storage vessels is limited by available heat supply, because of the endothermal nature of the hydrogen release reaction (typically in the order of 7.5 K cal/mol H.sub.2).
The development of suitable stacked assemblies using the ion exchange membrane fuel cell has been subject to various problems, mainly related to water and thermal management requirements.
Highly conductive ion exchange membranes have been available from E. I. DuPont Nemours and Company as perfluorosulfonic acid membranes known under the trademark NAFION. Dow Chemicals Co. has also developed high performance ion exchange membranes. Dow's U.S. Pat. No. 4,417,969 discloses membranes comprising a substantially fluorinated polymer with pendant chains containing sulfonic acid ion exchange groups. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water saturated conditions. Such a saturated condition, however, is difficult to maintain in stacked assemblies. Cell flooding, caused by buildup of product water in the stack, or membrane dehydration brought about by thermal gradients, or excessive water evaporation due to reactant gas flow, can each or all result in performance loss as well as membrane degradation.
Reactant gases have to be distributed over large electrode surfaces without resulting in local drying of the ion exchange membrane. Neither may the accumulation of product water interfere with the distribution of reactants.
Cell temperature is highly significant in this context. If cooling is not accomplished with a minimum temperature gradient across the electrodes, uniform current density will not be maintainable.
Suitable cell and system designs which address these problems are a prerequisite for the development of fuel cell power plants using the ion exchange membrane cell.
The problems of water management have been addressed by the first-named inventor herein in U.S. Pat. No. 4,175,165 in which hydrophylic coatings are applied to relevant structures in fuel cell assemblies to facilitate runoff of product water. The hydrophylic coating approach, however, is effective only in small cell configurations, and even there protection against cell drying is not adequate.
U.S. Pat. No. 4,215,183, assigned to General Electric Company, teaches use of a wet proofed carbon paper placed upon the catalytic electrode bonded to the membrane to prevent electrode flooding. Yet the approach of U.S. Pat. No. 4,215,183 did not resolve the problem of water removal from a cell assembly or membrane dehydration.
Accordingly, a need existed for an ion exchange membrane fuel cell and related stack thereof with an improved arrangement for heat and water management. It was also desired to present such a fuel cell stack which effectively makes use of hydride stored hydrogen for cell operation drawing on heat generated in cell operation for the liberation of hydrogen.